Talk:Crust (geology)

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[edit] Contribution enclosed

Excert from an essay I had to write for my degree about the Earth's crust.. It needs tidying up before being used in the main article.

Continental crust is divided into 2 layers, the upper and lower sections. The upper section is the only layer that can be sampled directly, through many samples of rocks from this layer it has been estimated that it is of a ?rock type between granodiorite and diorite? (Kearey & Vine, 1996). As it is too deep it has not been possible to sample the lower section of the continental crust using bore holes and such like. Instead the velocities of P waves, caused by seismic activity can be calculated, and from this the composition estimated. Early studies calculated the composition of this layer to be basaltic. However more recent studies have found that it is not possible for this lower layer to be basalt. This is because it has been shown that the pressure is around 2100MPa and the temperature is about 1100ºC(Green & Ringwood, 1967). Under these extreme conditions basalt would be expected to transform to ?ecologite via the intermediate stages of garnet granulite?. However these findings are disproved, as the velocity for P-waves in ecologite is around 8 km s-1, while the studied velocities in the lower crust varied from 6.5 to 7.6 km s-1 (Kearey & Vine, 1996). There is still much on going research and debate into the composition of the lower layer.

The line that divides the upper and lower sections of continental crust is called the Conrad discontinuity. This was detected due to the changes in the velocities of P-waves following seismic activity.

On average the overall depth of continental crust is considerably thicker than that of oceanic crust, being around 30 ?70km thick. It is thickest in high mountain ranges, such as the Himalayas and the Andes, this is due to the folding that took place during the time the mountains were formed. Due to its overall chemical composition it is less dense than oceanic crust.

Continental crust is formed along converging plate boundaries, such as the boundary between the Pacific and Indian plates. It is formed by any collision that involves two plates, whether oceanic or continental. This is because continental crust is less dense than oceanic crust, and therefore always goes over the top of oceanic crust at a collision zone.

Although some recycling and regeneration occurs of continental crust, it is of a significantly slower rate than that of oceanic crust. The most common form of recycling occurs at the margins between the two crust types, rocks that make up the continental crust are eroded, and carried off by the subduction of the oceanic crust. It is suggested that oceanic crust may act like sandpaper and break off bits of the continental crust, thereby recycling the continental crust.

Oceanic crust is a lot thinner than Continental crust, being only between 7 and 10km thick. It has been suggested that there are 3 distinct layers in oceanic crust, these have numbered 1, 2 and 3, with layer 1 being the top layer. Layer 1 has been investigated extensively due to its ease of access, it is only on average around 0.4km deep, and in some places, particularly around ocean ridges, does not exist. Layer 1 is made up of various sediments and clays. The second layer, Layer 2, varies between 1 and 2.5km thick, it has been sampled, by dredging at ocean ridges, where Layer 1 is not present. The results of these samplings have shown that it is igneous in origin and consists mainly of basalts.

The main depth of oceanic crust is taken up by Layer 3, this layer is believed to be mainly gabbroic in composition (Kearey & Vine, 1996). These are rocks which are similar to basalts. They were formed at depths where cooling and crystallisation have occurred slowly, this has created the coarse grains that they have (Whittow, 2000).

Oceanic crust is significantly younger than continental crust, this may be explained by subduction, as subduction continually recycles and recreates oceanic crust. Typically it is unknown to find oceanic crust older than around 180 - 200 million years old.

Reference: Kearey & Vine, 1996 Global Tectonics 2nd ed. Blackwell Science, Oxford --Cyr 10:51, 3 May 2005 (UTC)

Feel free to add pertinent data and the references to the article, also some may be useful in Continental crust and Oceanic crust as these are both stubs. Vsmith 15:22, 3 May 2005 (UTC)

Omphacite 18:43, 9 September 2007 (UTC)

[edit] Calcium oxide?

The table lists Calcium Oxide (CaO) as one of the major components of the earth's crust. Should this be Calcium Carbonate (CaCO3) instead, which is limestone? I don't think CaO exists in great quantities (if at all) naturally, since it reacts with water to form Ca(OH)2 and absorbs carbon diaoxide to form CaCO3.

Tcooke 05:01, 3 April 2007 (UTC)

Crustal composition is usually listed in terms of elements as oxides and the ref for this appears to be the 1911 Brittanica so may be somewhat dated. As for the calcium carbonate bit, limestone occurs as a part of the relatively thin sedimentary veneer on the typically igneous crust. The calcium within the more common igneous rocks occurs in the form of various calcium silicate minerals. So calcium silicate would predominate, however, as stated prev. the traditional way to list chemical compositions of rocks is by element oxide. After all that, I'd say we probably need to find a more current estimate or listing of crustal abundances. Vsmith 10:56, 3 April 2007 (UTC)

[edit] suggested major changes - comments?

The article partly duplicates and partly disagrees with separate entries for continental crust and oceanic crust. Perhaps only a brief summary of the Earth's crust should be in this entry, based on the present introduction.

Short additions would be useful for crusts of Mercury, Venus, our Moon, and Mars.

Such changes would drastically change this entry. Comments?

Also, updates for the separate entries on continental crust and oceanic crust would be valuable. Omphacite 18:43, 9 September 2007 (UTC)

[edit] major changes begun, please participate

add material for other planets, clean up and add to present text, and fix the table ... etc. Omphacite (talk) 00:12, 1 January 2008 (UTC)

Seems as 90+ percent of the linking articles are concerned with Earth's crust it seems that the emphasis should remain on earth with smaller sections for other planets. The section Perspective from the Moon is valid, but should follow the description of Earth's crust. Vsmith (talk) 21:30, 1 January 2008 (UTC)

A different order of the sections would be fine, but perhaps the text added to the introduction will make the present order seem better. Also, perhaps eventually the table on the composition of the continental crust might be moved to the entry on continental crust.Omphacite (talk) 22:15, 6 January 2008 (UTC)